The present invention relates to pneumatic boosters of the vacuum type comprising mainly a housing divided into two chambers by a movable wall structure, constructed of a substantially radial plate and a flexible diaphragm secured by its outer peripheral edge to the housing and forming a flange in the space between the plate and the housing, the plate being integral centrally with a hub structure housing a distribution valve device, and linked to a control member and to a controlled member, each by a swivel connection.
A booster of this type finds its application mainly in assisting the braking of automotive vehicles. In this application, the functioning of a booster of this type is well known and may therefore be explained briefly as follows: in the state of rest, the two chambers are connected to a source of reduced pressure; in the braking state, air under atmospheric pressure is admitted into one of the chambers; the difference in pressure between the two chambers then causes displacement of the movable wall and, as a result, assists braking.
U.S. Letters Pat. No. 4,117,769 describes a pneumatic brake booster in which a diaphragm is supported by a rigid plate allowing the extension thereof in the direction of displacement of the movable wall constructed of said plate and said diaphragm; during said displacement, said movable wall is not guided perfectly in the housing and, because of the swivel connections, the plate may become positioned askew and damage the diaphragm.
It is therefore an object of the present invention to obviate this drawback by providing a system which allows guidance of the movable wall inside the housing so as not to damage the diaphragm.
U.S. Letters Pat. No. 4,070,946 discloses a compressed-air screw jack in which the tubular piston has a guide skirt cooperating with the internal wall of the housing. In fact, the controlled member is not integral with the tubular piston which therefore has to be appropriately centered.
However, said elongated tubular piston is not subjected to forces which would cause it to become positioned askew, since it is not arranged between two swivel connections, and does not receive any control member which might apply an oblique force on it.